1. Field of the Invention
The present invention relates to a digital signal attenuation circuit for a compact disc--interactive(CD-I) player, and more particularly to a digital signal attenuation circuit which can digitally attenuate an audio signal read-out from a CD-I disc.
2. Description of the Prior Art
A CD-I player is a kind of reproducing apparatus which can read out and reproduce audio and video signals recorded on a CD-I disc in which the audio and video signals as well as audio and video information data are recorded, and which can control the output levels of the read-out audio and video signals by software, utilizing a mouse, a remote-controller, etc.
An audio signal attenuation circuit for a CD-I player can be grouped into an analog signal attenuation circuit and a digital signal attenuation circuit. The standard of attenuation required for such attenuation circuits is set to attenuate the audio signal by -1 dB within the range of 0 to -127 dB.
The analog signal attenuation circuit attenuates the analog audio signal which is converted from the digital audio signal obtained from the CD-I disc in accordance with 7-bit attenuation control signal from a system controller. Serial control type electronic volume control IC(IC No.: LC7535 manufactured by a Japanese company, Sanyo Electronic Co., Ltd.) is one of the conventional analog signal attenuation circuits, which can attenuate an analog signal by -1 dB within the range of 0 to -80 dB.
As shown in FIG. 1, a CD-I player incorporating such a conventional analog signal attenuation circuit comprises a microprocessor 10 as a system controller, an ADPCM (Adaptive Differential Pulse Code Modulation) decoder 30 for decoding an ADPCM audio data signal read-out from a CD-I disc to a digital audio signal, and a digital/analog(D/A) converter 40 for converting the digital audio signal from the ADPCM decoder 30 into an analog signal. A low-pass filter 50 for eliminating a high-frequency noise signal from the output signal of the D/A converter 40, an analog signal attenuator 60 for attenuating the output signal of the low-pass filter 50 by -1 dB within the range of 0 to -80 dB according to attenuation data from the microprocessor 10, and an amplifier 70 for amplifying the output signal of the analog signal attenuator 60 and outputting the amplified signal to a loudspeaker 80 are also provided.
According to the circuit as described above, the ADPCM audio data signal read-out from CD-I disc 20 is decoded to a digital audio signal by ADPCM decoder 30 under the control of microprocessor 10, and the decoded digital audio signal is converted into the analog signal by D/A converter 40. The analog signal is inputted to analog signal attenuator 60 through the low-pass filter 50 for eliminating the high-frequency noise signal from the analog signal, and then attenuated for a desired decibel(dB) value within the range of 0 to -80 dB by analog signal attenuator 60 under the control of microprocessor 10. The analog signal attenuated by attenuator 60 is outputted to the loudspeaker 80 after being amplified by amplifier 70.
However, such a conventional analog signal attenuation circuit as described above suffers from the disadvantage in that, since the conventional circuit is directed to the attenuation of the analog audio signal converted from the digital audio signal, much noise may be easily generated in the attenuated analog signal. Also, the analog signal attenuator should include a resistor array having a plurality of resistors to meet the required attenuation standard and a plurality of switches connected therebetween so that the analog audio signal may be attenuated by selectively turning on/off the switches in accordance with the attenuation control signal outputted from microprocessor 10, with the result that it is difficult to effect precise attenuation by -1 dB within the range of 0 to -80 dB since the performance characteristic of the resistors may easily vary due to a change in environmental conditions such as temperature, humidity and so on. Further, the analog signal attenuator requires extra peripheral circuits including a number of linear elements such as capacitors, operational amplifiers, resistors, etc., and thus the overall circuitry is greatly complicated.
Meanwhile, the conventional digital signal attenuation circuit comprises a memory receiving the 7-bit attenuation control signal from the system controller as an address signal thereof and storing attenuation data for attenuating the digital audio signal by -1 dB within the range of the attenuation standard; and a logic circuit for calculating the attenuation value of the digital audio signal using the attenuation data outputted from the memory. In this circuit, the attenuation data for 0 to -127 dB stored in the memory are outputted in accordance with the attenuation control signal from the controller and the logic circuit calculates the attenuation value of the digital audio signal by -1 dB utilizing the attenuation data from the memory.
However, the memory used in such a conventional circuit should be provided with a data table of about 127.times.8 bits in order to meet the attenuation standard of the CD-I player. Further, the logic circuit, which calculates the attenuation value of the digital audio signal utilizing the attenuation data from the memory, may be highly complicated in structure.